1. Field of the Invention
The present invention relates to preparation of powder granules by liquid condensation process and its manufacture of powder compacts.
2. Description of the Background Art
For a shaping of powders, a manufacture method should be properly selected according to the geometrical shape and size of a body to be shaped. A manufacturing method should be selected in consideration of the shape and size of a body to be shaped, and its adoption as a shaping method could be is substantially limited by the size, shape and their distributions of starting powders.
In a conventional art, a thermal spray drying method has been used to form free-flowing granules by artificially coagulating primary particles in the presence of organic additives. The thermal spray drying method is performed by spraying suspension droplets, containing starting powders, organic additives, and liquid medium, into hot air stream to obtain coagulated powder mass called granule consisting of primary particles and organic additives by removing liquid vehicle. In this process, since liquid vehicle is evaporated at the surface of suspension droplets, the liquid vehicle inside the suspension droplets should be moved to the surface through capillary pores formed between the primary particles.
If solid particles in suspension droplets have large differences in their sizes, fine particles are likely to move together with liquid vehicle through capillary pores formed between coarse particles (capillary migration) and to segregate at the surface of the dried granules. Likewise, organic additives also tend to migrate with liquid vehicle and to be concentrated at the surface similar to the fine particles.
Generally, compaction characteristics of granules are dependent on the packing structure of primary particles and the properties of organic additives, but the separation/segregation of component particles and organic additives by the capillary migration leads to macro-defect in dried granules and final shaped body.
The surface segregation of the organic additives and fine component particles provides a main reason to cause drying defects such as surface crust formation and hollow granules, which originates from rapid capillary migration at the initial stage of drying.
The aforementioned defects have great influences on the drying shrinkage of the suspension droplets during drying and on the packing density and strength of dried granules.
In fact, such defects of granules caused in the thermal spray drying have significant influences on the compaction and sintering behaviors of a dry-pressed body and also on the properties of the shaped and final sintered bodies. Especially, when the component particles in a composite system have large differences in density and size, or when the starting powder has a broad particle size distribution, the spray drying can readily result in various drying defects in dried granules. Thus, the uniformity of granules itself plays a critical role in determining the packing uniformity and structure of a shaped body.
The uniform granules can be obtained by eliminating or minimizing the separation of component particles and/or organic additives during removing liquid vehicle.
The microstructure of a shaped body and the property of a sintered body are determined by the uniformity and the physical property of the granules. The granules manufactured by the conventional thermal spray drying process can not avoid surface segregation of constituting substances and fine particles, morphological defects such as hollowness therein and variation of granule properties with granule size distribution.
One of effective methods to solve such shortcomings of the thermal spray drying is a freeze drying method, although it has a low productivity and is limited to aqueous suspension.